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A Flexible Photonic Crystal Material for Multiple Anti-Counterfeiting Applications Chang-Yi Peng, Che-Wei Hsu, Ching-Wen Li, Po-Lin Wang, Chien-Chung Jeng, Cheng-Chung Chang, and Gou-Jen Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b00292 • Publication Date (Web): 01 Mar 2018 Downloaded from http://pubs.acs.org on March 3, 2018

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ACS Applied Materials & Interfaces

A Flexible Photonic Crystal Material for Multiple AntiCounterfeiting Applications Chang-Yi Peng1, Che-Wei Hsu1, Ching-Wen Li1, Po-Lin Wang4, Chien-Chung Jeng4, Cheng-Chung Chang2**, and Gou-Jen Wang1,2,3* 1

Department of Mechanical Engineering, National Chung-Hsing University, Taichung 40227, Taiwan

2

Graduate Institute of Biomedical Engineering, National Chung-Hsing University, Taichung 40227, Taiwan

3

Ph.D. Program in Tissue Engineering and Regenerative Medicine, National ChungHsing University, Taichung 40227, Taiwan

4

Department of Physics, National Chung-Hsing University, Taichung 40227, Taiwan

*To whom correspondence should be addressed. Tel:+886-4-22840725 × 320; Fax: +886-4-22877170; Email:[email protected] **Co-corresponding author; Email:[email protected]

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Abstract In this study, a nano-imprinting method was introduced to fabricate polycarbonate films with transparent and flexible photonic crystal (FPC) structures. The fabricated flexible polymer films display a full-color grating due to the nano-hemispherical structures on the surface. Through the Bragg diffraction formula, it was confirmed that the FPC polymer films transfers part of the light energy to the second-order diffraction spectrum. Furthermore, the full-color grating properties can be modulated through geometric deformation due to the film’s elasticity. Additionally, anti-counterfeiting features were also successfully achieved when the polymer films were either engraved with drawings and bent; or patterned with fluorophores, which can be revealed under ultraviolet light. The most important aspect of this research is that the preparation of this FPC structured polymer film is inexpensive and convenient, enabling the mass production of a new generation of smart materials.

Keywords: nano-imprinting; flexible photonic crystal polymer; full-color grating; second-order diffraction; anti-counterfeiting; fluorophores

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Introduction Although pigments and dyes are generally used as coloring tools in the industry, they fade over time and often require additional treatment to ensure that the color is sustained. The field of biomimetics is the force behind the development of photonic crystal (PC) structures, which are structurally diverse in nature.

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The colors PC

structures provide are bright, adaptive camouflage and low-reflectance. Moreover, they are iridescent and cannot be mimicked by chemical dyes or pigments. In addition, as their coloration is generated by structural variation it is capable of persisting for a relatively long period of time. Therefore, PC structural colors are potentially useful in a variety of optical applications, such as color displays, sensors, diffraction gratings, and other industrial applications. 7, 8 A number of studies have been conducted on the use of PCs. For example, Hu et al. recently reported the use of PCs in anti-counterfeiting labels via their unique structural color changes; where magnetically induced self-assembly techniques were presented to form double photonic bandgap heterostructures.

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It has also been determined that one-

dimensional chain PCs can be applied to magnetically-controlled display units, photonic humidity sensors, reusable photonic paper, full-color photonic printing, and invisible photonic printing. Xu and his co-workers examined the optical properties and colors of reflected light on anodic aluminum oxide (AAO) and found that the reflected colors of aluminum oxide can be altered by changing the duration of immersion in phosphoric acid solutions.

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Furthermore, Haung et al. investigated the differences in color both before

and after removal of the aluminum oxide barrier layer and found that the reflected colors varied with alterations in the incident angle of light on the AAO nanostructure.

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The


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authors also examined carbon nanotubes@anodic aluminum oxide (CNTs@AAO) composite thin films developed by etching porous AAO before CNT growth and found that the color of composite thin films could be controlled by the thickness of the films. Structural chromogenic technology became the focus of research in 2014, and subsequently Montelongo et al. fabricated multicolor hologram silver nanoparticles via plasmonic scattering,

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and Olson’s group developed a full-color display technology

based on pixels, which combines tunable plasmon resonances of individual nanorods with diffractive coupling effects to achieve strong and sufficiently narrow optical resonances that produced vibrant RGB colors suitable for additive color displays. 13 Tan et al. applied a similar method to that of Olsen et al., in which 95 nm high nanorods were fabricated using negative-tone hydrogen silsequioxane (HSQ) and electron beam lithography. They then deposited 20-nm thick aluminum on the nanorods, which were fabricated with different diameters and spacing. 14 Results of the above studies demonstrate the effect of the periodic nanostructures on the reflected color, based on metallic plasmonic scattering. Bai et al. reported the formation of colloidal PCs by depositing silica nanoparticles onto design patterns via inkjet printing, wherein the color was controlled by adjusting the size and spacing of the silica nanoparticles.

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In addition, microbars can be used to modulate structural

coloration. Zhu et al. used a semiconductor photolithography process to fabricate ordered arrays of silicon bars on a flexible polymer and modulated the structural coloration by stretching the space between two adjacent bars according to the grating principle. Furthermore, Nam et al.

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created mono-layered, self-assembled PCs using inkjet

printing to exhibit structural colors and multiple colorful holograms for anti-

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counterfeiting applications. However, the structural colors of the fabricated device are relatively weak, and require an 80 W light source to clearly display the designed pattern. In summary, a PC is a periodic optical nanostructure in which the motion of photons is regulated via the pattern and size of the nanostructure arrangement. As PCs generate colors that cannot be imitated by chemical dyes or paints, they do not require an external driving force. That is why various nanotechnology methods have been developed to fabricate one-, two-, and three-dimensional PCs, as mentioned in the above literatures. However, the fabrication protocols of these nanostructures are time-consuming, which may also come from the use of expensive materials. Furthermore, a new PC structure needs to be constructed once a new pattern is desired, that is why commercial applications of three-dimensional photonic crystals have yet to be realized. In this paper, a simple method to facilitate the mass-production of FPCs with fullcolor grating properties is developed. The fabrication process of FPC is shown in Scheme 1; it involves the fabrication of an AAO template, nano-electroforming of a nickel nanomold and the subsequent nano-imprinting of FPC having structural colorations. As expected, the nano-imprinted FPC is capable of displaying full-color grating properties due to the nano-hemispherical structures on its surface. Moreover, multiple anticounterfeiting applications can be easily implemented based on the polymer’s intrinsic characteristics: surface curvature varies when the polymer film is bent or is coated with transparent substances of different surface tensions; the polymer film can be doped with (fluorophore) dyes or be patterned before nano-imprinting. In contrast to other works, the technique presented here does not require the fabrication of a new mold if a new pattern of FPC is desired. Thus, the total cost of the production process is greatly reduced

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because the FPC can be customized and mass-produced; as the Ni mold is reusable, which makes commercial applications highly feasible.

Scheme 1. Schematic of FPC fabrication procedure. The Ni mold is reusable and consequently, the as-produced FPC can be customized and mass-produced for multiple anti-counterfeiting (MAC) applications. Results and Discussion Principle of structural colors by photonic crystals-When light interacts with a grating structure, the diffracted light is composed of the sum of the interfering wave components radiating from the structure. In addition, the path length of light varies at any given point within the space through which the diffracted light passes. Therefore, the phases of the waves from each of the slits at each particular point will add or subtract from one another (through additive and destructive interference) to create peaks and troughs. The grating structure is therefore able to regulate the magnitude of the optical path and phase of diffracted light. Based on this principle, the periodic nano-hemisphere structure is used as a diffraction grating to split and diffract light into several beams traveling in different 6


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directions. Figure S1 shows a schematic illustration of light splitting on the surface of a nano-hemisphere array. Incident light irradiates the periodic structure, such as the nanohemisphere array, and the directions of the beams were determined using the diameter of the nano-hemisphere and the incident light wave; the coloration seen on the surface of the nano-hemisphere structure is thus a structural coloration. The relationship between the grating spacing and the angle of the incident and diffracted beams of light can be described by Bragg’s grating equation, 18

Λ(sinθ1 + sinθ2 ) = mλ

(1)

Where λ, θ1, and θ2 are the light wavelength, incidence angle, and the mth order diffraction–reflection angle, respectively, and Λ is the distance of the centers between two adjacent nano-hemispheres.

Fabrication and characterization of AAO, nano Ni mold, and flexible photonic crystal polymer (FPC)-Atomic force microscopy (AFM) images (shown in Figure 1(a)) were obtained to confirm that the nanostructure of the AAO template had been completely transferred to the nickel mold and that it had been copied to the polymer film after nano-imprinting. As listed in Table 1, the average roughness, diameter, and height of the nano-hemispheres on the AAO surface and the electroformed nickel mold were similar. Figure 1(b) confirms that the designed pattern had been successfully transferred from the AAO membrane to the nickel mold and then to the imprinted polymer film. Different colorations in each sample resulted from differences in incidence angles on different samples. In order to make sure that the nickel mold is reusable, the durability of

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the nickel mold was examined. Table 1 also listed the surface roughness, diameter of hemispheres, and height of the hemispheres of the nickel mold were measured after it had been used 20 times for nano-imprinting; the results were 45.1 ± 2.1, 433.4 ± 15.7, and 95.2 ± 5.5 nm, with variations of 7.96, 3.95, and 2.26%, respectively.

Figure 1. (a) AFM images of the AAO template, nickel mold, and nano-imprinted polymer, Insets: normal view images taken via digital camera (diameter of template is 1.7 cm); (b) colorful presentation of nano-structural films under indoor visible light, from right to left: the AAO membrane with a designed pattern, the corresponding nickel mold, and the imprinted polymer film FPC. Different colorations in each sample resulted from differences in incidence angles on different samples.

This reduction in dimensions can be attributed to residual polymer on the mold’s surface. SEM images of the AAO template and nickel mold are illustrated in Figure S2. Furthermore, as seen from three insets in Figure 1(a), the normal views of AAO template, nickel mold, and nano-imprinted polymer show no structural coloration (as a particular viewing angle is required to observe this). In nanostructure replication, previous studies have used oxidizing or polymer materials as the master mold to transfer the target

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structural morphology.

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However, it is not always possible to reuse these master

molds, as the mold can deform and the casting material may adhere to the mold; therefore, the lifetime of such a mold is relatively short. The electroformed nickel mold proposed in this study not only has better mechanical properties, but also has good acid/alkali resistance and enables duplication of FPC films using a more rigid substrate material. Table 1. Comparison of dimensions of nano-hemisphere structures on AAO surface, nickel mold, and nano-imprinted polymers. Surface Roughness (nm)

Diameter of Hemispheres (nm)

Height of Hemispheres (nm)

AAO

49.0 ± 1.4

451.2 ± 9.8

97.4 ± 2.6

Nickel Mold

53.2 ± 0.4

449 ± 10.5

98.3 ± 2.3

Imprinted polymers

50.7 ± 1.0

446 ± 13.1

93.6 ± 3.8

45.1 ± 2.1

433.4±15.7

95.2 ± 5.5

Used Nickel mold (20 X)

Basic structural coloration of nano-imprinted photonic crystal polymers-According to Snell’s law, the 0th order diffraction does not split colors but a high-order diffraction reflects light (with the direction determined by the incidence wavelength). In general, the diffraction efficiency is low on a polymer with the desired order, and thus a metal coating on the polymer grating can improve the diffraction efficiency of high-wavelength light in the visible region.

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Therefore, the nano-imprinted polymer surface was coated with a

10-nm Au thin film to enhance its diffraction efficiency. Figure 2 illustrates the structural coloration properties of the imprinted polymers. Figure 2(a) shows that the imprinted polymers can generate diffraction spectrum on the wall under white light LED irradiation. From the right to the left of the graph, blue, green, yellow and red colors are sequentially observed. This property indicates that the imprinted polymers can separate the

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polychromatic light into individual monochromatic light, similar to the characteristics of a white light diffraction grating. Figure 2(b) illustrates the structural colorations, varies gradually from blue to red, of nano-imprinted polymer surfaces at different viewing angles by fixing the light source. A detailed configuration of the structural colorations measurement setup is shown in Figure S3 and Figure S4 shows the structural colorations of the AAO template and the nickel mold. A digital video (DV S1) in the supplementary materials further illustrates the structural coloration variations. These results further demonstrate that the patterned PC structure of the AAO can be successfully transferred to a polymer using the simple nano-imprinting technique proposed in this study.

Figure 2. Structural colorations of the imprinted polymer in relation to different reflection angles. (a) the diffraction spectrum projected on wall under white light LED irradiation; (b) structural colorations of nano-imprinted polymer surfaces at different reflection angles; Diffraction spectrum of (c) AAO template; (d) nano-imprinted polymer surface; (e) the wavelength correlation plots from diffraction light vs. reflection angle; (f) the diffraction light path in the nano-hemisphere array.

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Table 2. The order of diffraction (m) estimated using Equation (1) Incidence angle (θ1)

Diffraction angle (θ2)

15 ∘

55 ∘

Measured wavelength of diffraction 426.67 ± 4.27

20 ∘

60 ∘

25 ∘

Calculated order (mc)

The order of diffraction (m)

1.126808

1

478.00 ± 11.89

1.127172

1

65 ∘

544.00 ± 23.32

1.089524

1

30 ∘

70 ∘

578.33 ± 26.99

1.110271

1

35 ∘

75 ∘

604.67 ± 26.72

1.135525

1

40 ∘

80 ∘

629.00 ± 37.34

1.154066

1

45 ∘

85 ∘

670.00 ± 9.98

1.133839

1

50 ∘

90 ∘

681.67 ± 7.130

1.155480

1

*Λ = e = 446 nm

A detailed configuration of the diffraction spectrum measurement setup is shown in Figure S5. Figures 2(c) and 2(d) show the diffraction spectrum of the AAO surface and the nano-imprinted polymer surface, respectively. From the AAO diffraction spectrum, with an increase in the diffraction angle (θ2), the structural color gradually changes from blue to green (λ = 500−600 nm) and then to red (λ = 600−700 nm). The polymer surface diffraction spectrum shown in Figure 2(d) coincides with that of the AAO template shown in Figure 2(c). The diffraction spectrum ranges only from 55 to 90°; no structural coloration can be detected when the sample is almost parallel with the light source and the reflection angle (θ)

approaches 90°. Organizing data of Figure 2(c) and Figure 2(d),

it can be seen that the wavelength of observed diffraction light is positively proportional to the diffraction angle, as shown in Figure 2(e), where the diffraction spectrums of the AAO template and its corresponding polymer surface are seen to be very similar. To further explain the structural coloration properties, a schematic of the light path of the

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nano-hemisphere array is shown in Figure 2(f), in which θ⊥ denotes the normal direction of the grating surface, e is the notch spacing, and θ1 and θ2 denote the incident and the diffractive angles relative to θ⊥, respectively. In a normal diffraction situation, the phenomena of reflection and diffraction occur simultaneously. When incident and reflective light are ipsilateral of the normal, the optical path difference is ∆ =cb + bd , where cb =e × sin θ 2 and bd =e × sin θ1 ; therefore, ∆ is the right item (mλ) of the Bragg’s grating equation in Equation (1). Substituting each measured data set of λ, θ1, and θ2 into Equation (1), the order of diffraction (m) for each data set was calculated as shown in Table (2). The order of diffraction calculated using Equation (1) confirms that ∆ = λ for each measured data set. The diffraction spectrums shown in Figure 2(a), (c), and (d) also indicate that the photonic crystal structure of hemispheres can absorb the light of most visible wavelengths at a specific incidence angle, resulting in reflected wavelengths which are the result of diffraction and interference of visible light inside the photonic crystal. This phenomenon resembles the spectral characteristics of a grating. According to Equation (1), if the diffraction order (m) is zero, no spectral effect is produced. Hence spectral effect must rely on high-order diffraction (m= ±1, ±2, ±3…). The calculated orders of diffractions tabulated in Table 2 confirm that the photonic crystal structure of hemispheres can transfer part of the light energy to the first order diffraction spectrum.

Anti-counterfeiting features of flexible photonic crystal structures-The preliminary anti-counterfeiting feature of our nano-imprinted FPC films can be achieved based on the characteristic of photonic crystal, by coating different transparent substrates onto the 12


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polymer surface. For example, water/glycerol (3/7, v/v) droplets in Figure 3(a) and water droplets (b) on the surface of FPC with different curvatures, and hence inducing different light diffraction patterns, are illustrated. Brilliant color bands are observed on the droplet with respect to the background of FPC, especially the low curvature droplet (Figure 3b). The principle of the curvature-induced discoloration can be depicted using the schematics in Figure 3(c) and Figure 3(d). The water droplet on the nano-imprinted polymer surface converts the parallel incident beams into various lights with different angles of refraction, thereby displaying the full-color spectrum. As shown in Figure 3(c), analysis based on the schematic shown in Figure 2(f), when the diffraction incident angle θ’1 is less than θ1, the same observed angle θ2 will result in a smaller λ value in Equation 1; resulting in blue shifted colors to display on the surface of the droplet. Alternatively, once the angle θ’1 is larger than θ1, the same observed angle θ2 will result in a bigger λ and hence the red shifted colors are revealed (as indicated by the blue and red arrows in Figure 3(b)). Furthermore, in the θ’1 > θ1 case, a bigger θ’1 as well as a bigger θ2 will bring about a large λ value which makes the second order diffraction spectra possible (mλ, m=2 in Figure (3a)). That is, a larger curvature enables a larger range of refraction angles, and hence is more likely to induce second-order diffraction, as shown in Figure 3d. To summarize, a transparent substrate with a higher curvature may increase the diversity of incidence angles, hence narrowing the full-color spectra region. Therefore, more diverse colors, including second order spectra, can be observed from the surface. On the other hand, since most of the energy is still in the zero and first order diffraction spectra, only a small amount of energy is distributed to the second order diffraction spectrum, that is why the structural colors of the second order diffraction spectra in Figure 3(a) are bleaker.

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Based on the discussion above, the as-prepared FPC is possible for watermarking anticounterfeiting applications.

Figure 3. Surface diffraction pattern observation of FPC with (a) larger curvature from water/glycerol (3/7, v/v) droplets; (b) smaller curvature from water droplets, the obsevation angle was kept at θ2=30° while incident light angles θ1 at 15, 30, 60, 85°. (c) mechansim of the curvature-induced discoloration and (d) the second order diffraction spectrum.

Since it has been concluded that variation of surface curvature may affect the diffraction spectra, another more direct way to change the surface curvature is by bending the material. By utilizing the flexible property of the polymer film, an alternative anticounterfeiting feature can be obtained by bending the FPC (Figure 4). The bent FPC structure receives the parallel incident light at various angles resulting in variable

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diffraction light with different wavelengths (Figure 4(a)). It was observed that a small bending of 10° is sufficient to induce discoloration. When the membrane was bent to 30°, the structural colors are compressed to the upper half of the membrane with a more pronounced banded distribution. The lower half of the membrane remains transparent and exhibits no diffraction phenomenon. Adopting this optical property, an additional anticounterfeiting feature as shown in Figure 4(b) can be further implemented. When the membrane is not bent, the photonic crystal structure colors cover the pattern on the back of the membrane. After bending, part of the membrane surface does not produce structural color, hence the pattern on the back of the membrane can be observed.

Figure 4. Effect of geometrical deformation on structural color change of printed polymer membranes. (a) the relationship between the structural color change and the curvature of the membrane, (top row) side view, (bottom row) top view; (b) additional anti-counterfeiting feature of printed polymer membranes, (top) unbent membrane, (bottom) 20° bending membrane. A DV (DV S2) in the supplementary materials further illustrates the bending controlled anti-counterfeiting feature.

Further anti-counterfeiting applications were explored with fluorescence imaging method; where the fluorophore is patterned on to the backside of the nano-imprinted polymer film to enable observation under a UV-light source. A practical method for

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enabling anti-counterfeiting observation was determined and the configuration of the measurement setup is the same as in Figure S3, but with the UV-light source fixed above the film. Different structural colorations under visible light irradiation with different reflection angles are clearly illustrated in Figure 5(a). The sample simultaneously shows both structural coloration and fluorescent patterns under visible light and UV-light irradiation, as shown in Figure 5(b); thereby confirming its secondary anti-counterfeiting features. Further anti-counterfeiting features can be achieved by coating a customized fluorescence pattern on an imprinted polymer film. It is possible to preload the customized fluorescence into a polymer film and then nano-imprint to fabricate FPC. Thus, photonic crystal structured flexible polymers having different optical properties can be made through combinational fabrication procedures, such as dye (fluorophores) doping, surface tension dependent material coating, and film bending.

Figure 5. Extra anti-counterfeiting feature where N-Bu and 3,5 DAB fluorophores (green and red emission colors, respectively) were patterned onto the back of the nanoimprinted polymer film and a hand-held UV lamp with 365nm long wave UV-light output is used as the UV-light source. (a) Under 3W LED white light irradiation only; (b) under 3W LED white light and UV-light irradiation, simultaneously.

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Conclusions In this paper, a mass production method that requires no additional re-casting is proposed as a feasible solution for nano-imprinting flexible photonic crystal polymers with diverse structural coloration and multiple anti-counterfeiting features, including viewing polymer surfaces at different reflection angles, coating transparent substance with different surface tensions, simple bending, and integration with fluorescence. Traditional fabrication of photonic crystal structures has typically required complex and expensive nano-fabrication processes. This study presents a relatively simple and costeffective method for producing photonic crystal structures based on nano-imprinting. Our experimental results demonstrate that by using a nickel mold it is possible to use nanoimprinting to produce flexible photonic crystal polymers with a full-color grating property. The unique chromogenic principles of the nano-imprinted polymer surface are very difficult to duplicate using existing techniques, and has the advantages of being achieved with a low cost and can be mass-produced with customization.

Materials and Methods Multiple anti-counterfeiting polymer fabrication: The sequential fabrication procedures (as shown in Scheme 1) involved in creating the proposed multi anti-counterfeiting polymer based on the structural coloration of photonic crystal are described below. AAO template fabrication: An AAO membrane was used as the template for nanoelectroforming a nickel nano-mold for nano-imprinting. The AAO template was prepared using a specially designed anodizing process. Pure (99.9995%) aluminum foil (128-µm thick) was cleaned with acetone, ethanol, and deionized (D.I.) water in sequence, 17



followed by annealing at 400 °C for 3 h. To polish the foil surfaces, electropolishing was performed using a 1:4 perchloric acid and anhydrous ethanol solution as the electrolyte under a constant voltage of 20 V for 2 min. The AAO film with parallel nanochannels (diameters of around 175 nm and thicknesses of 13 µm) was obtained by anodizing the aluminum foil in a 5-wt.% phosphoric acid solution under an applied voltage of 180 V at 0°C for 3 h. The remaining aluminum was then dissolved beneath the barrier layer in an aqueous CuCl2 ・HCl solution, which was prepared by dissolving 13.45 g of CuCl2 powder in 100 ml of 35-wt.% hydrochloric acid solution; this then revealed the honeycomb barrier-layer surface of a convex hemisphere array with an average diameter around 450 nm. Nano-electroforming: The nano-electroforming process included the following: (1) Electrolyte preparation: A sulfamate bath containing nickel sulfamate tetrahydrate [Ni(NH2SO3⋅4H2O)] and nickel chloride [NiCl4⋅6H2O] in a boric acid solution [H3BO3] was implemented for electrodeposition. Since the nickel sulfamate tetrahydrate possesses low internal stress and high solubility in water, the concentration of Ni ions in solution can be increased. Hence, the rate of electrodeposition can be enhanced. (2) Electroforming: A 10 nm of Au thin film was sputtered onto the barrier-layer surface of the fabricated AAO membrane to serve as the working electrode for electroforming. Electrodeposition was then conducted

using a nano-electroforming system (EGG Instruments

Corporation/Model 263A) with a bulk nickel anode and the gold thin film-coated AAO template as the cathode, under a positive voltage with a setting of 10 mA/cm2 to the

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anode. The processing time depends on the desired thickness of the Ni film. After deposition, the AAO template was etched by a NaOH solution (1.0 M) to obtain a Ni mold of complemental concaves of the original AAO template. Nano-imprinting and demolding: Nano-imprinting is a useful technique for massproduction of nanostructures.

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In this study, nano-imprinting was adopted for mass-

production of FPCs. Polycarbonate was used as the substrate for flexible photonic crystal films. The nano-imprinting was performed using a customized machine at 185°C under a pressure of 0.15 MPa for 12 min, followed by cooling to room temperature. Demolding was then conducted to obtain the flexible photonic crystal film. Fluorophore doping: (Figure 5) The fluorophores 4-(6-(1-methyl-4-piperazinyl)-1, 3dioxo-1H-benzo

[de]isoquinolin-2(3H)-yl)

butane

(N-Bu)

25

and

3,5-Bis(4-

methoxyphenyl)-4,4-difluoro-8-(4-methoxyphenyl)-4-bora-3a,4a-diaza-s-indacene DAB)

26

(3,5-

were used as the green and red fluorophore patterned onto the nano-imprinted

polymer film, respectively. The compound N-Bu (absorption maxima at 390 nm) exhibits stable luminescence properties in most organic solvents (487 nm in toluene to 515 nm in H2O) as well as at a solid state (510 nm), with quantum yields ranging from 0.4 to 0.6. The compound 3,5-DAB (absorption at 410 nm and 580 nm (maxima)) also exhibits stable luminescence properties in most organic solvents (615 nm in toluene to 620 nm in H2O) as well as at a solid state (615 nm), with quantum yields ranging from 0.4 to 0.6. In this study, the fluorophore patterned polymer film was easily fabricated via painting the fluorophore-containing solution (methanol) onto the back of a nano-imprinted polymer film.

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ASSOCIATED CONTENT Supporting Information. The following files are available free of charge on the ACS Publications website at DOI: Figure S1~S5 present the light splitting on the nano-hemisphere surface; structure of the barrier layer surface of a fabricated AAO membrane; the setup for structural colorations measurement; structural colorations of the AAO membrane and the nickel mold; diffraction spectrum measurement, respectively. AUTHOR INFORMATION Corresponding Author * [email protected] ** [email protected]

Acknowledgements The authors would like to offer their thanks to the Ministry of Science and Technology of Taiwan under grant number MOST-104-2221-E-005 -028 -MY3 for their financial support of this research.

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Table of Content A Flexible Photonic Crystal Material for Multiple Anti-Counterfeiting Applications Chang-Yi Peng1, Che-Wei Hsu1, Ching-Wen Li1, Po-Lin Wang4, Chien-Chung Jeng4, Cheng-Chung Chang2**, and Gou-Jen Wang1,2,3*

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Flexible Photonic Crystal Material for Multiple ... - ACS Publications

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